The current network access technologies are mainly based on asymmetric digital subscriber line (ADSL) and cable modem. More and more fiber to the home (FTTH) access networks are also under construction in recent years. Passive optical network (PON) is a broadband access network that is rapidly gaining popularity recently. The network architecture of PON is usually a tree-based topology, with a terminal device at one end of the tree, called optical line terminal (OLT), and the other end of the tree including a plurality of branches with a user device at the end of each branch, called optical network unit (ONU). OLT and ONU of PON are connected through passive elements. The downlink packet transmission of PON is in broadcast mode, i.e., the packet from OLT may be received by all ONUs.
The uplink packet transmission is in time division multiple access (TDMA) manner. PON must have appropriate control mechanism to avoid the collision caused by the packets sent by two or more ONUs at the same time. To improve the network utilization, a dynamic bandwidth allocation (DBA) method is used to allocate bandwidth to each ONU.
The bandwidth allocation of TDMA PON is basically the OLT polling the ONUs or the ONUs using piggyback to add the backlog data to be transmitted to the end of the upstream packets to the OLT. However, as the distance between OLT and ONU may be as long as 30 km, the single-round polling may take several hundreds of micro-seconds (μ sec) in signaling. With piggyback and multiple users online simultaneously, two successive piggyback turns for the same ONU may also be as high as few milli-seconds (msec).
Distributed controlled PON usually uses carrier sense multiple access/collision detection (CSMA/CD) technology. FIG. 1 shows a schematic view of an exemplary distributed controlled PON system. In the exemplary distributed controlled PON system in FIG. 1, downstream data is carried by wavelength λ2, and the signal is split at star coupler (SC) 101 to be broadcast to all ONUs. Upstream data is carried by wavelength λ1, and the signal is broadcast at star coupler (SC) 101 to OLT and all ONUs. Therefore, all ONUs may listen to λ1 to know whether other ONUs are transmitting upstream data and to avoid the collision from occurring.
There is still possibility that a plurality of ONUs find that no other ONUs are transmitting and decide to start transmission at the same time. With this scenario, each ONU may use an exponential backoff algorithm to determine a random delay to avoid the collision.
FIG. 2 shows a schematic view of another exemplary distributed controlled PON system. In the exemplary distributed controlled PON system in FIG. 2, remote node 201 includes N 2×2 couplers. Coupler k uses one output for transmitting upstream data from ONU k to OLT, and the other output for transmitting signal to next ONU k+1. In this manner, each ONU may listen to know whether the previous ONU is transmitting upstream data, and may start transmission when the previous ONU finishes transmission to avoid collision. In other words, each ONU must listen to hear that the previous ONU starts to transmit, wait until detecting the previous ONU finishes transmission, and then the ONU may start to transmit.
Because ONU is customer premises equipment (CPE), ONU has the right to determine when to switch on and off. Therefore, if ONU k of PON in FIG. 2 switches off, ONU k+1 will not be able to hear any data transmission of ONU k. In this scenario, ONU k+1 may keep waiting and the entire network may keep waiting and no ONU can start transmission.
FIG. 3 shows a schematic view of yet another exemplary distributed controlled PON system. In the exemplary distributed controlled PON system in FIG. 3, the remote node has a 3×N splitter 301 for broadcasting upstream signal to each downstream ONU. Through splitter 301, each ONU may receive messages to other ONUs. This PON system uses two channels, 1550 nm channel for transmitting downstream signal from OLT to ONUs, and 1310 nm channel for transmitting upstream signal from ONUs and broadcasting control signal to other ONUs. In other words, 1310 nm channel is a shared channel for upstream data and control data. The ONU system needs two high speed receivers for receiving 1550 nm channel and 1310 nm channel, and a high speed transmitter for transmitting on 1310 nm channel. Because the control and the ONU upstream share the same channel, each ONU still must wait until all ONUs finish transmitting before starting the new state broadcasting.
In current PON, the centralized network access control is entirely controlled by the OLT in the ONU network access time and access duration. The ONU cannot notify the OLT about the data amount to be transmitted. In comparison with ONUs in the distributed controlled PON system, the bandwidth utilization is low, the delay is high and no quality of service is guaranteed.